The present invention relates to an induction device and, specifically, to an induction device that operates on electromagnetic induction and is suitable for use, for example, as a transformer of an insulated power converter.
In an insulated power converter, electric power is converted by a transformer. Japanese Patent Application Publication 2010-153724 discloses a switching power supply device that is one of insulated power converters. The switching power supply device has a coil substrate structure formed by including a first coil substrate having a primary transformer coil part and a second coil substrate disposed overlapping the first coil substrate and having a secondary transformer part. In the coil substrate structure, the primary and secondary transformer coil parts include spirally extending conductor patterns, respectively as seen in the thickness direction of the substrate. The first and second coil substrates are disposed one on the other in such an overlapping relation that the primary and secondary transformer coil parts coincide each other as seen in the substrate thickness direction.
A hybrid vehicle has been put into practical use, whose drive wheels are driven by a motor at a start and in a low speed range and by an internal combustion engine in intermediate and high speed ranges to reduce fuel consumption and exhaust gas emission. Such a hybrid vehicle has a main battery supplying power with a voltage of 200 to 300 volts to a traction motor. However, the voltage of 200 to 300 volts needs to be stepped down to 12 volts to drive electric auxiliary equipment of a vehicle whose rated voltage is usually 12 volts.
When the numbers of turns of primary and secondary coils of a transformer are represented by N1, N2, respectively, and the voltages of the primary and secondary coils of the transformer by V1, V2, respectively, N1/N2=V1/V2 is true if all magnetic flux from the primary coil passes through the secondary coil. Furthermore, N1/N2=k*(V1/V2) is true if a rate k of all magnetic flux from the primary coil passes through the secondary coil. The rate k is called coupling coefficient between the primary and secondary coils. The rate k has a value of 1 or less. If all magnetic flux from the primary coil passes through the secondary coil with no leakage of the magnetic flux, the rate k is 1. Therefore, in stepping down the voltage of 200 to 300 volts to 12 volts, through depending on the value of rate k, the number of turns of the primary coil need be 10 times or more than that of the secondary coil.
There is a limit to the spaced distance between any two adjacent turns of coil that is made by using a method such as pressing or etching. Increasing the number of turns of the coil causes the coil to be enlarged in the radial direction thereof. On the other hand, there has been a demand for increasing the number of turns of the coil without enlargement in size to meet the increasing consumption of power.
The present invention, which is made in view of the above problems, is directed to an induction device that can increase the number of turns of coil without enlargement in size.